The membrane potential of a cell is always close to the equilibrium potential of:
A) Faraday’s constant.
B) the least permeable ion.
C) the most permeable ion.
D) the electrical gradient.
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The resting membrane potential of a neuron is its membrane potential at rest, i.e. when it is not firing action potentials or receiving input from presynaptic neurons.
Actually, the use of the word “potential” here is not entirely accurate from a physics point of view. The term “membrane potential” does not actually refer to the electric potential at any point in the neuron. Rather, it refers to the difference in electric potential inside and outside the neuron. So, when we say that a neuron’s resting potential is -70 mV, this means the electric potential inside the cell is 70 mV lower than outside the cell. For this reason, membrane potential is more accurately thought of as “membrane voltage” or “potential difference across the membrane”.
In this MCAT post, we’re going to explain why the resting membrane potential (or membrane voltage) of a neuron typically hovers around -70 mV. In order to do so, we’ll first have to cover a few seemingly irrelevant topics, topics which include the sodium-potassium pump and ion equilibrium potential.
If we know the ion concentrations inside and outside of the neuron, we can calculate the equilibrium potential of a given ion. Equilibrium potential for a given ion is the membrane potential at which there is no net movement of that ion across the cell membrane. Equilibrium potential for a given ion can be calculated using the Nernst equation, shown below in Figure 1.
Here Eion is equilibrium potential, [ion]o and [ion]i are the concentrations of the ion of interest outside and inside the cell respectively, R is the gas constant, F is Faraday’s constant, z is ion valence (the charge of the ion of interest, e.g. +1 for sodium and -1 for chloride), and T is temperature.
On the MCAT, you likely won’t be doing calculations using the Nernst equation since you won’t have access to a calculator. However, you should understand, qualitatively speaking, how the concentration gradient of each ion influences that ion’s equilibrium potential.
For instance, since sodium has a high concentration outside the neuron and a low concentration inside the neuron, sodium ions “want” to move into the neuron. But remember that equilibrium potential is the membrane potential at which there is no net movement of an ion across the membrane. So, if we assume that there is no net movement of sodium across the membrane, we must also assume that the inside of the neuron is positively charged compared to the outside. (To resist the tendency of sodium, a positively charged cation, to flow down its concentration gradient into the neuron, the inside of the neuron must be positive compared to the outside.) Hence, the equilibrium potential of sodium is positive, with a value of roughly +60 mV as given by the Nernst equation.
Using the same logic, we can see that since potassium’s concentration is high inside the neuron and low outside of it, its equilibrium potential must be negative, with a value of about -90 mV according to the Nernst equation.
Ok so now that we’ve discussed the sodium-potassium pump as well as ion equilibrium potential, we’re ready to explain why the typical resting membrane potential of a neuron hovers around -70 mV.
We’ll begin by stating that even though the sodium-potassium pump is constantly importing K+ into the neuron, there is little actual net movement of that ion across the cell membrane when the neuron is at rest. This is because the pumping of K+ into the neuron by Na+/K+ ATPase is countered by the movement of K+ out of the neuron via leaky potassium channels, which are open when the neuron is at rest.
Now with that said, let’s recall the definition of equilibrium potential. As stated above, the equilibrium potential of an ion is the membrane potential at which there is no net movement of that ion across the cell membrane. We’ve already established that there is little net movement of potassium across the membrane of a neuron at rest, so this must mean that the resting membrane potential approximates the equilibrium potential of potassium. This value was stated above as -90 mV, which is close to -70mV but not exactly equivalent. The reason there’s a slight difference is because other ions (sodium, as well as chloride and calcium) are also slightly permeable at rest. This means that resting membrane potential is also slightly affected by the equilibrium potentials of those other ions.
Still, resting membrane potential (-70 mV) is pretty close to potassium’s equilibrium potential (-90 mV). And, again, this is explained by the fact that potassium is the most permeable ion when the neuron is at rest.
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